How to enter a document in mongodb with size larger than 16MB?

None, see https://www.mongodb.com/docs/manual/reference/limits/.

And thankfully because I feel there are real issue with your schema design if you end up with 16Mb structured documents to store. Media files, I understand, but structured data, I would like to ear about the use cases.

Every time you will update your 16Mb documents, you will eventually write 16Mb on disk.

Here another link https://www.mongodb.com/docs/ops-manager/current/schema-advisor/reduce-document-size/